Invading earth for minerals (or even water) and getting any resistance would be foolish on "their" part. Any mineral you can think of is available freely on many planets throughout the solar system and throughout our galaxy, and probably in larger quantities than here, and with no resistance. If an alien race calculated that they might lose even a handful of their numbers to us unruly, savage humans, they could just as easily find all those elements (and water) elsewhere. So why bother?